Design guide IDP2303(A)
LLC design
Application Note
34
Revision 2.0
2017-05-03
Δ𝑓
𝑠𝑤
Δ𝑉
ℎ𝑏𝑓𝑏
= −
𝜆
𝑡
𝑠𝑤
2
Note: Similar to the LLC transfer function, the VCO transfer function depends on the VCO steady-state
switching period,
𝑡
𝑠𝑤
. Usually, the VCO transfer function has a very high gain in order to cover the whole
switching frequency range.
4.3.4.3
LLC regulation loop
The LLC regulation loop is based on external analog circuits and is controlled by the TL431 regulation
network. The configuration is shown in Figure 22 below.
Figure 22
Analog control loop configuration
Assume the impedance of
𝐶
1
,
𝐶
2
and
𝑅
3
network is
𝑍
𝑓
. The small signal transfer function is
𝐺(𝑠) = −𝐺
0
1 +
𝜔
𝑧1
𝑠
1 +
𝑠
𝜔
𝑝1
where
𝐺
0
=
𝑅
𝑢𝑝
𝑅
1
𝑔
𝑜𝑝𝑡𝑜
𝜔
𝑧1
= 𝑠
𝑍
𝑓
𝑅
2
𝜔
𝑝1
=
1
𝑅
𝑢𝑝
𝐶
3
𝑔
𝑜𝑝𝑡𝑜
is the current transfer rate of the opto-coupler, and
𝐶
3
is the external capacitance plus the parasitic
capacitance of the opto-coupler which is around
2 𝜇𝐹
. The equivalent impedance
𝑍
𝑓
is
𝑍
𝑓
(𝑠) =
(𝑅
3
+
1
𝑠𝐶
1
) ⋅
1
𝑠𝐶
2
(𝑅
3
+
1
𝑠𝐶
1
) +
1
𝑠𝐶
2
Substitute this into
𝜔
𝑧1
,
